Corrosive liquids, such as strong acids and bases, and highly reactive chemical solutions, are frequently used in chemical processes. Liquids also may be abrasive or erosive, due to, for example, suspended solids in the liquid. Valves are particularly vulnerable to both corrosive and erosive environments due to moving mechanical parts of the valve and the necessity of maintaining the integrity of a fluid seal. Valves have been lined with various types of materials such as rubber, glass, plastic, and the like, to combat either a corrosive or erosive environment. For example, to provide corrosion resistance, iron valves may be lined with glass, or various rubbers such as butyl rubber, hard rubber, natural rubber, neoprene and the like. For erosion resistance, iron valves may be lined with or composed of hardened steel, or the like. Also, valves may be lined with various kinds of plastics (such as TEFLON.TM. or KYNAR.TM.) or fiberglass-reinforced plastic. Lead has also been used to line valves, and many valves are coated with a thin layer of glass, which is generally applied in the form of low-temperature melting frit and then heated to form the glass lining in place.
While many lining materials are suitable for corrosive or erosive environments, most are not suitable for environments that are both erosive and corrosive, such as liquid acids containing suspended abrasive solids. Metal valves lined with rubber, glass, lead and, the like, are commercially used in these environments that are both corrosive and erosive, but the lifetime of these valves is relatively short, particularly where the fluid stream is moving at high velocities. None of these lining materials have a long life in handling a high-velocity, corrosive, and erosive liquids.
Ceramic materials are generally known for both corrosion and erosion resistance. Valves made of a ceramic material, however, can be difficult to form and consequently are relatively expensive. Furthermore, while ceramic devices can be corrosion and erosion resistant, they generally possess poor flexural stress resistance and impact toughness, so that a valve formed completely of ceramic could be easily fractured.
A popular design of valves for used in corrosive and erosive environments is of the so-called diaphragm valves. For example, U.S. patent application Ser. No. 07/637,365 discloses a diaphragm valve with a ceramic lined valve body. These valves have a straight-through bore with an elastomeric diaphragm that is extendable into the bore through a side opening to block the passage of fluid through the bore. These valves are very corrosion and erosion resistant as the valve bodies are completely lined with high-purity ceramic materials. However, since the diaphragm is of a flexible material, it is naturally not of the same corrosion resistant ceramic material. Corrosion resistant elastomeric materials for use in the diaphragms are known in the art; but generally the more corrosion resistant materials, particularly fluorinated elastomeric polymers such as teflon, have a lower elasticity and flexibility. In a straight-through valve the diaphragm must commonly be extended and stretched to an extent greater than is possible for the more corrosion-resistant elastomeric materials. Therefore, these more corrosion-resistant elastomeric materials cannot be used in these valves.
In order to solve this problem of insufficient elasticity of some diaphragm materials, diaphragm weir valves have been developed. Weir valves allow the use of diaphragm materials that have low elasticity and flexibility. In a weir valve a weir is disposed in the flow channel opposite the diaphragm. The weir partially blocks the flow channel, but provides a sealing surface on the top of the weir that is closer to the diaphragm than would be the case of a straight-through valve that has no weir. When the diaphragm is in the retracted position, fluid flows up and over the weir. When the valve is closed the diaphragm only extends to and seats against the top of the weir to provide a seal, instead of the diaphragm extending completely into and across the bore as in straight-through valves. Thus, in order for the valve to close completely, the diaphragm is not required to extend and stretch as far as in a straight-through valve.
Matsutani U.S. Pat. No. 3,349,795 discloses a diaphragm weir valve with a complex ceramic valve casing. The ceramic valve casing is a complex one-piece design shaped to fit within a complex valve body. The valve body and casing are shaped such that a arcuate top surface is formed to provide a weir curving downwards to a circular opening. The complex shape of Matsutani ceramic casing limits the method for manufacturing the shape mostly to slip casting methods. For this reason, such a complex shape is not conducive to fabrication from high-purity, high density ceramics materials, which on a commercial scale, requires fabrication by powder compaction methods, particular isostatic and uniaxial compaction methods. In addition, common machining methods, such as lathe forming, milling, cylindrical grinding, and surface grinding, cannot easily be used to form the shape because of its complexity. The ceramic materials that can be made into the shape of a Matsutani casing by slip casting methods are usually of a porcelain variety and typically of low purity, and do not have the corrosion resistance for many environments, particularly to highly caustic solutions. In addition, the complex shape of the Matsutani ceramic casing requires a specially constructed two-piece valve body so that the casing may be inserted into the valve body.
An additional problem with complex ceramic shapes, such as in the Matsutani reference, is that flexural stresses are inevitably induced in the ceramic shapes by pressure fluctuations, pipeline excursions, flange misalignment, and the like, which cause these shapes to be quite fragile. In addition, such one-piece complex shapes are subject to thermal flexural stresses due the differing thermal expansion coefficients of the ceramic and the metal of the valve material.